Organic light-emitting devices (OLED) are becoming increasingly important in lighting and display applications. However there are still significant improvements yet to be made for OLED technology that may help to encourage widespread use. For example, to replace a conventional light source with a OLED device, it may be helpful to raise power efficiency of OLED to a level that can compete with the level of conventional light sources. Generally, power efficiency is about 60-90 lm/W for fluorescent lamps. Therefore, it is desired to attain an efficiency of at least about 60 lm/W to help white OLED to compete as replacements for fluorescent lamps. The United States Department of Energy (DOE) described a 2015 target benchmark of about 150 lm/W (assuming CRI>80 and CCT=2700-3000K). Thus there is a need to further improve device efficiency.